Conventional radio receivers implement a mixer for frequency conversion of a signal from a desired frequency to an intermediate frequency. The intermediate frequency is the difference between an incoming carrier frequency and the local-oscillator frequency. These radio receivers, however, have to attenuate an undesired spurious response due to the fact that the mixer will convert two frequencies, the desired frequency and its image frequency, to the intermediate frequency. Generally, tuned filtering and careful selection of the intermediate frequency are used to reduce this undesired response.
An alternate way to reduce the spurious image response uses an image reject mixer. Image reject mixer frequency converters usually generate two signals, each with a desired signal component and an image signal component. The two desired signal components are in phase and the two image signal components are 180 degrees out of phase. When the two signals are summed together, the desired signal components add and the image signal components cancel. Generation of these phase-shifted image signals is typically accomplished with balanced-to-unbalanced transformers (baluns), tuned circuits, hybrid couplers, or surface acoustic wave (SAW) technology, none of which can be easily implemented on an integrated circuit (IC) without using an expensive trim process.
The inability to integrate an image reject mixer on an IC increases the number of components in the receiver, increases the size of the receiver and its production costs, and restricts the overall design of the printed circuit board used for the device. Thus, there is a need for a frequency conversion mixer that rejects the spurious image response yet can be integrated in an IC.